Promotion of Niobium Oxide Sulfidation by Copper and Its Effects on Hydrodesulfurization Catalysis

Niobium sulfide (NbS2) has shown a promising performance in versatile applications, but its formation from Nb oxide is thermodynamically limited, which hinders its usage. We predicted, based on thermodynamic calculations, and experimentally verified that the addition of copper (Cu) to niobium promotes Nb oxide sulfidation at practical temperatures. A series of bimetallic bulk NbCu structures at varying Cu/Nb molar ratios were synthesized via a coprecipitation technique. X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR) results revealed that copper facilitated sulfidation and reduction of niobium oxide. The synthesized NbCu catalysts were evaluated in hydrodesulfurization (HDS) of dibenzothiophene (DBT) at 325 °C and 3 MPa. Copper promotes sulfidation but does not change the turnover frequency of surface NbS2 and behaves as a spectator. The optimal Cu/Nb molar ratio was found to be 0.3, below which there is not enough Cu to ensure maximum sulfidation and above which copper segregates to the catalyst surface and blocks NbS2 active sites. The weight-based sulfur-removal activity of the optimal catalyst was doubled in the presence of copper. This study demonstrates that the bimetallic Earth-abundant NbCu catalyst could be a promising candidate for hydrotreating catalysis. Since Cu-promoted NbS2 was determined to be more active than molybdenum sulfide per mass and surface area, the copper addition may be recommended for thermodynamically limited niobium oxide sulfidation to promote NbS2 formation as a potential alternative to MoS2 for a variety of emerging applications with transition-metal sulfides. The study also demonstrates that the concept of promotion in catalysis can be extended to the assisted increase of the number of active sites, with no effect on their performance during catalysis.